Early Reflections in Home Theater Rooms: Beneficial or Detrimental?

Originally published: April 7, 2007

No, I am not sitting in my home theater at 6:00 a.m., ruminating about how the pattern on the wall from the lighting sconce looks different from the back row. This article is about sound reflections. Specifically, early sound reflections in home theaters. I have been wondering whether there are better objective measures to help with addressing early reflections; specifically to help determine (a) whether they are problems and (b) what to do about them if they are. To begin to address these issues, it might be beneficial to examine the state-of-the-science. Floyd Toole pointed out in a recent JAES article that the acoustical design of small spaces has its origins “in investigations of performance spaces—concert halls and auditoriums” [1].

Toole goes on to question this logic and, in my opinion, his skepticism is well-founded. How many home theaters sound like concert halls? How many concert halls sound like home theaters? Is either one capable of sounding like the other? Is that even a desirable design basis? The answers to these (hopefully) rhetorical questions would seem to indicate that any metric used for a large space would, at the least, need to be modified for use as a small room metric. I have argued for a number of years—along with others—that the large room metrics we’re accustomed to should not be used in small room design.

Borrowing Richard Rives Bird’s love of home theater/kitchen comparisons [2], consider the following: When baking a cake, it not be prudent to measure out the amount of salt in fractions of a gallon. Nor would it make sense to calculate out fuel economy in miles per teaspoon. These may seem like exaggerations, but consider the volumetric ratio of a concert hall and a home theater can typically be on the order of 700:1. Know how many teaspoons are in a gallon? Answer: 768. Shall we choose to equip our kitchens with measuring spoons or 768-teaspoon jugs? Similarly, shall we design our home theaters using RT, or shall we consider some other metrics that more accurately ascertain the relevant time domain characteristics of a significantly smaller space?

In our recently added YouTube video we discuss room acoustics and the importance of balancing the sound in your room to get the best possible performance from your speakers.  Over treating can make your listening space sound too dead and take the life out of the room.  Too little treatment can result in a room whose sound is dominated by reflections  resulting in a very blurred, echoey and fatiguing environment.  In a home theater room you should still be able to carry on a normal conversation comfortably but still have a room that is dampened enough to allow the first arrival of sound to be much louder than the associated reflections.  Using multiple subs will even out the bass and negate expensive and bulky bass traps and low end acoustical treatments that reduce system efficiency and often look unsightly.  Watch our video to learn more on this topic and feel free to ask your questions on our forum or YouTube channel.

How to Properly Treat Room Acoustics for a Home Theater Listening Space

The answers to these (hopefully) rhetorical questions would seem to indicate that any metric used for a large space would, at the least, need to be modified for use as a small room metric.

What about Reverberation Time?

Reverberation time (RT) has a long and glorious history in room acoustics. Starting with Sabine [3] and working through to more recent works like those of Beranek [4], we have volumes (excuse the pun) of empirical data showing what constitutes “good” and “bad” for concert hall and auditorium acoustical design, with many presentations of RT data. Armed with this sort of evidence and a good scientific brain, one could conceivably design a fantastic concert hall using nothing more than RT as a measuring stick. Considering organ music? Target the RT in the 3.0 to 5.0 second range for the midband frequencies and the room is probably well on its way to aural ambrosia.

But home theaters don’t benefit from this approach. Using only a target midband RT of 0.30 to 0.50 seconds for a home theater does not guarantee a good sounding room.

Editorial Note on Volume / Time Relationship

Some may point out an apparent discrepancy between the 700:1 volumetric ratio and the 10:1 target RT ratio. The relationship between volume and time in acoustics is equivalent to the relationship between volume and distance. Knowing this, and that the cube-root of 700 is roughly 9, the discrepancy should be resolved.

Once we start looking at the decay of sound to 1/100ths of a second, our design parameters begin to border on the ridiculous. If the measured RT is within 10% of the target RT in a home theater design—a typical RT design target for concert halls—the result carries far less weight than the RT resulting from a similar design target in a concert hall. In other words, if the target RT is 3.0 seconds in a concert hall and the result is 2.7 seconds, the room should sound more or less as intended. If the result is 4.0 seconds (a 33% overshoot), then the room probably does not sound as intended. Conversely, if the target RT for a home theater is 0.30 seconds and the result is 0.27 seconds, who cares? If the result is 0.40 seconds (same 33% overshoot), again, who cares? Regardless of the result, the room could sound good…or bad!

To examine some alternatives to RT, besides the metrics and methods covered quite thoroughly in Toole’s journal article [1], I delved into the work of Yoichi Ando. One of the parameters he has established as a metric for concert hall design evaluation is called the “effective duration of the envelope of the normalized auto-correlation function” [5]. No, I am not making this up. For the purposes of this discussion, I will abbreviate this to τ_e, which is the same abbreviation Ando uses. Despite the verbose description, I believe τ_e to be a useful metric. It is basically a measure of the repetitiveness or the reverberation contained in a source signal. Its relationship to preferable room acoustics qualities and quantities is derived from subjective tests. The τ_e of the source material can be used in designing spaces that are intended to reproduce that source material. According to Ando, speech signals generally carry a τ_e value of about 10 milliseconds (ms). What this ultimately means in terms of the science is beyond the purview of this article. For comparison, slow classical music pieces tend to have a longer τ_e, generally in the range of 100-200 ms.

Measurements – Subjectivists and Objectivists Unite

Armed with this information, I went about measuring the τ_e of some movie dialogue just to get a sense of whether Ando’s estimate for the τ_e of speech was accurate for my intended application. Using some software packages available from Yoshimasa Electronic Inc. called “Realtime Analyzer RAD” and “Sound Analyzer RAD” [6], I was able to process an excerpt of center-channel dialogue from the Sam Gamgee character (played by Sean Astin) in the second LOTR movie [7]. I measured a τ_e between approximately 4.7 and 12.2 ms.

The next step was to dig deeper into what Ando had to say about room design based on τ_e. Ando’s specialty is large spaces. However, much of Ando’s research is based around correlations between objective metrics and (subjective) listener preferences. This is in stark contrast to many other architectural acoustics teachings that revolve around room function. Ando’s subjective research has led to methods that are used to match listener profiles with concert hall seats based on some simple screening tests. Since his studies are heavily weighted towards the subjective, I wondered whether his metrics would be more applicable to smaller spaces than the traditional objective metrics like RT. The first thing I investigated was something that actually involved RT. (Drat.) One of the basic “rules-of-thumb” Ando developed was that the RT should be roughly 23 times greater than τ_e. From my measurement of the LOTR dialog, this means Sam Gamgee would likely sound the best in a room with an RT between 0.11 and 0.28 seconds. If I were to go against my own philosophy and actually use RT to design a home theater room, this range of RT values for the midband frequencies would probably be on the low side, but not unreasonable.

To Treat or Not to Treat – First Reflections

However, I would prefer not to rely on a large room metric. Exploring Ando a bit more, he spends a lot of time on the “preferred delay of a single reflection,” or, more practically, the delay associated with the first reflection arriving at a listener’s ears. This would be synonymous with the initial-time-delay gap, or ITDG, from Beranek [4]. To abbreviate, I will again borrow from Ando and use Δt₁ for the delay of the first reflection. When applied to speech signals, Ando found empirically that there is beautifully simple relationship between the τ_e of the source material, the preferred Δt₁, and the relative pressure amplitude of the first reflection. Once again using the results of my dialogue analysis above, the result is that the preferred Δt₁ for the LOTR excerpt averages out to be roughly 10 ms, but depends on how much treatment there is on the surface providing the reflection. The less treatment, the lower the preferred Δt₁ will be. The more treatment, the higher it will be. For small rooms, this translates to how far the “best” seats are from the nearest surfaces, most notably the side walls. A good designer should be able to apply some basic math and gauge easily whether the listener will prefer treated or untreated side walls.

To illustrate this using my LOTR dialogue analysis, the ideal Δt₁ for untreated side walls (say, gypsum wallboard) could be approximately 4.8-12.5 ms [eqn. (4.1) from ref. 5]. Similarly, if absorbers are to be considered for the side walls, the ideal Δt₁ could be in the 6.1-15.9 ms range. Recalculating again for diffusive walls could provide an optimum Δt₁ of 5.3-13.7 ms. These ranges can be compared to the geometry of a room to figure out what the most appropriate early reflection treatments for the side walls might be. For example, if the average listening distance (DL, from the center channel) is 10 feet and the width of the room (W) is 15 feet, Δt₁ works out to be roughly 7.1 ms. Based on our calculations above, any solution—reflection, absorption, or diffusion—could result in a subjectively pleasing result at the listening positions. For a smaller room where DL is 5 feet and W is 10 feet, Δt₁ works out to be approximately 5.5 ms. In this case, reflection or diffusion might be the preferred treatment options, but absorption may not be desirable. Finally, changing DL again to 13 feet and W to 25 feet will give us a Δt₁ of about 13.4 ms. Hence, absorption and/or diffusion, but not reflection, might be the best treatment to consider. The above is summarized in the Figure. A simplified delay calculation is presented in the upper portion of the Figure. Once the Δt₁ is calculated, it can be compared to the line graph on the lower part of the Figure. Possible treatment methods can then be evaluated. A spreadsheet is also available for download that can further simplify the process of treatment selection.

Small Room Considerations

The above is not intended to be used for the design of an actual home theater. I have simplified Ando’s methods and made some rather large assumptions, chief among them being that Ando’s methods and techniques for room design can actually be applied to small rooms. I think they can be, but that’s my opinion. This may seem counterintuitive to some—it certainly flies in the face of the conventional wisdom on treating early reflection in small rooms, which has traditionally been approached with the “absorb the heck out of everything” mindset. However, the writings and researches of folks such as Toole [1], Moulton [8], and others, would seem to indicate that the conventional wisdom may not always be wise, and is certainly becoming less conventional with the advent of better sound reproduction technology (loudspeakers, processing, etc.), center-channel dominance of movie dialog, the ubiquity of heavy carpet (and pad) and “acoustical” ceiling tiles in many home theaters, etc. The potential desire for early side wall reflections is real. Careful consideration of the objective measure of the source material being considered for reproduction in a home theater—and the fact that the faithful reproduction of soundtrack dialogue is (arguably) the #1 goal of the home theater—would seem to indicate that this “new” school of thought is not far off the mark when it challenges the use of heavy absorbers on the side walls. That is not to say everyone should tear down their absorbers. For larger listening rooms, a certain amount of absorption and/or diffusion is necessary. However, for smaller spaces (and equally smaller budgets), the conventional “necessity” for side wall absorption (or diffusion) is debatable. The absorption of early reflections in small rooms is largely a carryover of conventional home studio design (and this is not errant). The wealth of literature and online advice concerning the acoustical treatment of small sound studios will make it difficult for any paradigm shift for home theater acoustics to take hold.

Fabric-wrapped panels and absorber systems are relatively poor off-axis absorbers.

Additionally, it is not surprising that the home theaters that have been treated with side wall absorption—and there are many—are perceived as good-sounding rooms. If one considers that the overwhelming majority of absorptive panels used in home theaters are of the “fabric-stretched-over-fuzz” variety, then the widespread acceptance of this technique falls right in line with what has been discussed here. Fabric-wrapped panels and absorber systems are relatively poor off-axis absorbers. That is, when measured off-axis, there is less absorption. When a person is sitting at a listening position with reflections arriving at some angle from the side wall treatments, the panels are actually reflecting a good deal of sound. This could easily lead to a pleasant sounding environment that falls neatly in line with what I’ve discussed above. The absorption is working best on-axis, resulting in absorption of many reflective artifacts, such as flutter echoes, that would otherwise decrease sonic quality. However, when looking at one crucial element, that of the arrival of the first reflection, the “fabric-stretched-over-fuzz” absorbers are worse performers, much to the benefit of the average home theater.

I really haven’t delved into anything groundbreaking or “new” here. I’ve simply taken another approach to reaffirming the work of others. The main point is that absorption may not be the only option for treating the average home theater. There is certainly ample research indicating that leaving the early reflections alone, or treating them mildly with some diffusers of “fabric-stretched-over-fuzz”-type panels, would not yield poor results. In fact, many of the “bass traps” folks are selling could serve the purpose of controlling low-end and midrange problems, but reflect enough sound to the main listening area so that the objective results are consistent with the subjective expectations. (Indeed, this is already the case in many home theaters.)

Early reflections may or may not be major problems in home theaters. Addressing them through the use of heavy absorption should be pursued with caution. I believe the issue of early reflections and their relative merits (or lack thereof) in any home theater should never be ignored. It is my hope that continued discussion and further research on subjective preferences in small acoustic spaces will result in the development of better objective measures. New and better metrics or methods—possibly including applications of Ando’s work similar to what I’ve discussed above—would help home theater designers provide more definitive answers to the questions about early reflections.

Acknowledgment

Thanks to Erikk Lee at Auralex Acoustics, Inc. for extracting the LOTR center channel audio for use in my analyses, and to Masatsugu Sakurai of Yoshimasa Electronic Inc. for his wonderful programs and his assistance in making them do what I wanted them to do.

References

1. “Loudspeakers and Rooms for Sound Reproduction—A Scientific Review” by Floyd E. Toole, Journal of the Audio Engineering Society, Vol. 54, No. 6, June 2006.

2. See, for example, A New Way to Think About Room Acoustics

3. Collected Papers on Acoustics by Wallace Clement Sabine, 1992 Peninsula Publishing republication of the 1922 Harvard University Press publication. Available from http://asa.aip.org/publications.html.

4. Concert and Opera Halls: How They Sound by Leo Beranek, Acoustical Society of America, 1996.

5. Architectural Acoustics: Blending Sound Sources, Sound Fields, and Listeners by Yoichi Ando, Springer, 1998.

6. See http://www.ymec.com/.

7. DVD: The Lord of the Rings: The Two Towers, Chapter 50, “The Tales that Really Mattered…”, ©MMIII New Line Home Entertainment, Inc.

8. “The Significance of Early High-Frequency Reflections from Loudspeakers in Listening Rooms” by David Moulton, 99^th AES Convention (1995), Preprint No. 4094.

About Jeff

Jeff D. Szymanski is an acoustical engineer with over 20 years of experience in the music, room acoustics, and noise control industries. He is a member of the Audio Engineering Society, the Acoustical Society of America, the Institute for Noise Control Engineering, and Synergetic Audio Concepts. He co-holds two U.S. patents for acoustical treatment devices, is an accomplished musician, and is a resident of the beautiful state of Kansas.